Ultrahigh permeance of a chemical cross-linked graphene oxide nanofiltration membrane enhanced by cation–π interaction

Cross-linking with large flexible molecules is a common method to improve the stability and control the interlayer spacing of graphene oxide (GO) membranes, but it still suffers from the limitation of low water flux. Herein, a novel high flux GO membrane was fabricated using a pressure-assisted filtration method, which involved a synergistic chemical cross-linking of divalent magnesium ions and 1,6-hexanediamine (HDA) on a polyethersulfone (PES) support. The membrane cross-linked with magnesium ions and HDA (GOHDA–Mg2+) exhibited a high water flux up to 144 L m−2 h−1 bar−1, about 7 times more than that of cross-linked GO membranes without adding magnesium ions (GOHDA), while keeping excellent rejection performance. The GOHDA–Mg2+ membrane also showed an outstanding stability in water for a long time. The effects of magnesium ions on the GOHDA–Mg2+ membrane were analyzed using several characterization methods, including Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results indicated that magnesium ions not only promoted reasonable cross-linking, but also improved the stacking of GO sheets to give lower mass transfer resistance channels for water transport in the membranes, resulting in the ultrahigh permeance of the GO membranes.


Introduction
Graphene oxide (GO) is a two-dimensional network with a thickness of one atom, which has great potential in the eld of water treatment because of its excellent hydrophilicity, 1 remarkable stacking property 2 and other unique properties. 3 In GO membranes, a large number of oxygen-containing functional groups, such as hydroxyl, epoxy, carboxyl and carbonyl, are randomly distributed outside the pristine graphitic sp 2 region. 4 These oxygen functional groups act as water nanochannel spacers to introduce water molecules into the sp 2 region which can allow water molecules to ow without resistance. 5 Therefore, GO-based membranes are considered as the next generation of nanoltration membranes.
In the practical application of GO nanoltration membranes, the interlayer spacing between the neighboring GO nanosheets, water ux, efficient rejection and stable performance of the membrane are all crucial in water purication. 6,7 However, the GO membrane is prone to swelling in water due to the large number of hydrophilic oxygen functional groups, which is not conducive to the stable performance of GO membrane in practical application. 8 There have been many effective efforts to stabilize the interlayer spacing and prevent the swelling tendency of GO membranes. For example, by crosslinking with organic large molecules and ions, 6,[9][10][11] as well as by reducing the GO membrane to decrease the interlayer spacing, the stability is signicant improved. 7,12 Despite the great progress, these membranes still suffer from limitation of low water ux (<27 L m À2 h À1 bar À1 ). 2,13,14 The decreasing of the interlayer spacing and excessive cross-linking between sheets, not only decrease the water channel, 7,10,11 but also increase the mass transfer resistance, 9 leading to the low permeance. Therefore, GO membranes is still under the expectation, which requires further increase of water ux without sacricing stability and rejection. 2 These challenges hinder the potential applications of GO membranes in water purication.
In our previous work, accurate cationic control of the interlayer spacing of GO membranes withÅngström precision using ions, has been achieved. 11 The existence of cations adsorbed on GO surface, can greatly improve the atness of the GO nanosheets, which is conducive to the stacking of GO sheets to form reasonable water nanochannels. 11,15 Therefore, chemical crosslinking together with ions, potentially promoting reasonable cross-linking and improving the water channels of membrane in terms of atness and surface with low mass transfer resistance.
For Mg 2+ , it is the most divalent cation and abundant in seawater, which has the same strong cation-p interaction with the graphene sheets as the high multivalence metal ions (Fe 2+ , Co 2+ , Cu 2+ , Cd 2+ , Cr 2+ and Pb 2+ ) have. 16 Interestingly, GO membranes controlled by Mg 2+ ions have the largest interlayer spacing compared with other metal ions in seawater. 11 The large interlayer spacing in GO membrane is the prerequisite for ultrahigh water permeation. However, due to strong cation-p interaction between Mg 2+ and GO akes, Mg 2+ ions can be adsorbed on the GO surface during the cross-linking process, which can prevent excessive chemical cross-linking and improve the water channels of membrane in terms of atness and low mass transfer resistance surface during the crosslinking reaction.
In this study, the GO membrane with high water permeance for dyes rejection was prepared by pressure-assisted ltration method, which was a synergistic chemical cross-linking of divalent magnesium (Mg 2+ ) ions and hexamethylenediamine (HDA) (GO HDA-Mg 2+ ) on a polyethersulfone (PES) support. We also prepared the cross-linking GO membrane only by HDA for comparison (GO HDA ).

Preparation graphene oxide (GO) suspension
Graphene oxide (GO) was prepared from commercial graphite powder by a modied Hummers' method as previously reported. 11,17 Graphite powders were rstly pre-oxidized by concentrated H 2 SO 4 , K 2 S 2 O 8 , and P 2 O 5 solution with continuous stirring for 4.5 hours. Secondly the mixture suspension was washed by DI water and vacuum drying for a night. Then, oxide graphite was further oxidized in concentrated H 2 SO 4 and KMnO 4 , diluted with a lot of DI water. 30% H 2 O 2 is further trickled to remove excess KMnO 4. The product was separated by centrifugation and washed with 1 : 10 HCl aqueous solution and DI water. Finally, the GO suspension was prepared aer half an hour of ultrasound.

Preparation cross-linking GO membrane
MgCl 2 were added into 150 mg L À1 GO and stirred evenly. The concentration of Mg 2+ in the mixed suspension was 0.25 M, 0.125 M, 0.05 M, 0.025 M and 0 M, respectively. Next, 1,6-hexanediamine was dissolved in the above mixed suspension to prepared 0.075 M aqueous solution. The mixtures were le to rest overnight at room temperature, 3 mL solution was dissolved to 40 mL, respectively. Then, they were ltered through a polyethersulfone (PES) membrane under a pressure of 1 bar. Finally, the membranes were washed with 1 : 10 HCl aqueous solution and DI water. The membranes prepared at a series of concentrations of Mg 2+ were named as GO HDA-0.25Mg 2+, GO HDA-0.125Mg 2+, GO HDA-0.05Mg 2+, GO HDA-0.025Mg 2+ and GO HDA , respectively. GO HDA-0.25Mg 2+, GO HDA-0.125Mg 2+, GO HDA-0.05Mg 2+ and GO HDA-0.025Mg 2+ are collectively referred to as GO HDA-Mg 2+ .

Filtration experiment
The permeation and rejection performance of the cross-linking membranes were tested by using a vacuum lter system with an effective membrane area of 11.34 cm 2 . The rejection tests were performed with 10 mg mL À1 rhodamine B (RB), pararosaniline (PR), and methyl blue (MB) solutions, respectively. The molecular weights of the three dyes are 479.01, 323.82 and 319.86 g mol À1 , respectively. When ltration went steady, the water ux (J w ) and Rejection (R) was measured at 1 bar by using the following eqn (1) and (2): where J w is the water ux (L m À2 h À1 bar À1 ), V is the volume of the permeation water (L), A is the effective membrane area (m 2 ). Dt is the permeation time (h) and the P is the ltration pressure (bar). C p and C f are the concentration of permeation and feed dye solution which were measured by ultraviolet spectrophotometry, respectively.

Results and discussion
The chemical properties of GO and cross-linking GO membrane The FT-IR spectra of the GO, GO HDA-0.25Mg 2+ and GO HDA are shown in Fig. 1b. The chemical structure of GO was clearly changed by HDA. The FTIR spectrum of the pristine GO suggested the presence typical vibrations, such as, the hydroxyl C-OH (stretching at 3594 cm À1 ), carbonyl C]O (stretching at 1730 cm À1 ), carboxyl -OH (bending at 1418 cm À1 ), aromatic (stretching vibrations at 1622 cm À1 ) and epoxy C-O (stretching at 1020-1227 cm À1 ). [18][19][20][21] For GO HDA-0.25Mg 2+ and GO HDA, the peaks of hydroxyl, epoxy and carboxyl decreased dramatically aer cross-linking with HDA, and a new absorption peak was observed at 1550 cm À1 which represent the bending vibration of N-H. 9,10,22 The reduction of hydroxyl, epoxy and carboxyl groups and the new generation of amine conrmed that HDA likely reacts with these oxygen-containing groups to form C-N covalent bonds, during the condensation reaction of HDA with hydroxyl 23 and carboxyl 24 and the nucleophilic addition reaction of amine with epoxy. 24 In addition, compared with the GO HDA , the GO HDA-0.25Mg 2+ has more oxygen-containing groups and less amine groups, indicating a weaker reduction and lower degree of cross-linking due to Mg 2+ added in the reaction process.
To further reveal the chemical properties of GO membranes, C 1s of XPS spectra were used to analyze the elemental compositions of the chemical bonds. As shown in Fig. 1d-f, the deconvoluted C 1s spectra were divided into four peaks at binding energies of 284.6, 286.7, 287.8, 288.9 eV, which corre- Importantly, a new peak appears at 285.5 eV representing the C-N bond, 9,10 which are 4.13% and 14.51% for GO HDA-0.25Mg 2+ and GO HDA , respectively. The results demonstrated that the GO HDA-0.25Mg 2+ has an effective cross-linking similar to GO HDA , while a weaker reduction and lower degree of cross-linking, which is consistent with our FT-IR spectra results. It indicates that under the interaction of Mg 2+ , it is benecial to reasonable cross-linking between GO and HDA. It not only ensures the stability of the membrane, but also facilitates the formation of channels with low mass transfer resistance.
In addition, we used XPS to detect the atomic concentrations. Fig. S2b † shows the survey XPS scans of the prepared cross-linked membranes. We can see that there are no observable Mg 2+ ion signals. During the ltration, the ltrates were collected when the ltration process went steady (aer about 20 min), which can help to rule out the adsorption effect by the membrane. Thus, the high rejection for dyes remained constant with increasing membrane thickness, is mainly due to stable size exclusion effect based on stable chemical cross-linking with HDA and the water channels of membrane improved by Mg 2+ during the cross-linking reaction.
Effects of Mg 2+ on the interlayer spacing of cross-linked GO As mentioned above, the water channel of membrane is an important parameter for permeation. These membranes were further analyzed by XRD. There were clear shis of the interlayer spacing (indicated by the Bragg peaks of XRD) relative to the GO membrane that had been immersed in pure water, as shown in Fig. 1c. Immersion in pure water resulted in a GO membrane spacing from 8.5Å to 12.8Å, consistent with early reports. 11 In contrast, the shis of interlayer spacing of GO HDA and GO HDA-0.25Mg 2+ between dry and wet state were smaller. The interlayer spacing of GO HDA were 9.3Å and 11.9Å in dry and wet state, respectively, due to the limitation of newly formed C-N bonds between the GO sheets. 9,10 Similarly, those of GO HDA-0.25Mg 2+ in dry and wet state were 9.1Å and 11.4Å. However, as shown in the inset of Fig. 1c, the full width at half maxim (FWHM) of GO HDA-0.25Mg 2+ is obviously narrower than that of GO HDA , indicating that GO HDA-0.25Mg 2+ has better uniformity of the water channel, than that of the GO HDA membrane. Thus, the channel can be shown in the schematic of Fig. 1a. The rippling and wrinkled structure was aligned attened by Mg 2+ , which is conducive to the stacking of GO sheets to form a surface with low mass transfer resistance for water transport. 11,15 Therefore, it can be predicted that the GO HDA-0.25Mg 2+ membrane has a relatively high permeance while maintaining high rejection.

Morphology of the GO and cross-linking GO
The Atomic Force Microscope (AFM) image of GO akes was observed, as shown in Fig. S2a. † The thickness of GO monolayer is about 0.96 nm. Fig. 2a-f show the SEM images of surface and cross-section morphology of the GO, GO HDA and GO HDA-0.25Mg 2+ membranes. The thickness of the cross-linked membrane was about 200 nm, while the thickness of GO membrane was 150 nm. These SEM images showed that the resulting thin layered membrane was continuous and free of macro pores or defects, which is critical for a highly efficient separation process. 11 As shown in Fig. 2f, GO HDA-0.25Mg 2+ membrane is obviously a multi-layers structure like GO membrane. As shown  Fig. 2g, we also evaluated the stability of the membrane immersed in water. The GO membrane disintegrated aer 20 min without mechanical stirring, and then seriously dispersed aer 60 min. But GO HDA-0.25Mg 2+ and GO HDA still remain stable, even when the water was stirred by glass rod, indicating that GO HDA-0.25Mg 2+ membrane can overcome the swelling problem in water like GO HDA .

Permeance of cross-linking GO
We performed dyes permeation tests to verify the water ux and the reject rate of GO, GO HDA and GO HDA-0.25Mg 2+ membranes. As shown in Fig. 3a-c, HDA resulted in a decrease in water ux for rejecting methylene blue, pararosaniline and rhodamine B from 12.5 L m À2 h À1 bar À1 , 30.6 L m À2 h À1 bar À1 and 23.5 L m À2 h À1 bar À1 for GO membrane to 11.2 L m À2 h À1 bar À1 , 10.8 L m À2 h À1 bar À1 and 13.4 L m À2 h À1 bar À1 for the GO HDA membrane, respectively. But it didn't make much difference to the reject rate. The performance of GO HDA is consistent with early reports. 9,10 In addition, we also observed the performance of the GO membranes controlled only by Mg 2+ (GO Mg 2+ ), which prepared with the same experimental process as did the crosslinking experiments, as shown in Fig. S3. † The water ux of GO Mg 2+ for rejecting methylene blue was 39 L m À2 h À1 bar À1 , which is only slightly higher than that of GO. In contrast, GO HDA-0.25Mg 2+ membranes have ultrahigh water ux, which were 143.2, 114.4 and 144.2 L m À2 h À1 bar À1 for the three dyes, respectively, while still rejected dyes as those of GO HDA . Interestingly, the water uxes are nearly 10 times higher than those of GO HDA membranes without sacricing dyes rejection, as shown in Fig. 3d. It further demonstrated that GO HDA-0.25Mg 2+ membrane has much better uniformity and lower mass transfer resistance than that of GO HDA . We also listed the separation performance of GO-based membranes previously reported for organic dyes. As shown in Table 1, GO HDA-0.25Mg 2+ showed great advantage on the water ux.
The stability of water ux and dyes rejection of GO HDA-0.25Mg 2+ and GO HDA membranes was analyzed. The uxes were measured for 2 h and recorded every 10 min aer adding DI water into the feed side. As shown in Fig. 3d-f, the uxes of the GO HDA-0.25Mg 2+ and GO HDA were about 114.4 L m À2 h À1 bar À1 and 11.2 L m À2 h À1 bar À1 , respectively, which were very stable during the whole ltration process compared with the ux varies from 35.7 L m À2 h À1 bar À1 to 4.3 L m À2 h À1 bar À1 of the GO membrane. It demonstrated that GO HDA-0.25Mg 2+ and GO HDA all have the outstanding stability in ltration process, which is attributed to the C-N bond formed between GO and HDA. The performance of the GO membranes, which were synergistically cross-linked by K + (GO HDA-K + ) or Fe 3+ (GO HDA-Fe 3+ ) were also observed, as shown in Fig. S4. † The membrane cross-linked with K + and Fe 3+ ions and HDA (GO HDA-K + and GO HDA-Fe 3+) exhibited water ux of 40.1 L m À2 h À1 bar À1 and 140.6 L m À2 h À1 bar À1 , respectively, while keeping >99% rejection for methylene blue, indicating high multivalence metal ions are benecial to permeability. Compared with these tested metal cations, the Mg 2+ ions still have the greatest advantage in terms of water ux.

Effect of thickness and concentration of Mg 2+ on permeance of cross-linking GO membrane
It is clear that Mg 2+ can greatly improve the permeance of the cross-linking membrane. Considering potential inuence of Mg 2+ concentration on cross-linking, we prepared GO membranes with a series of concentrations (from 0.25 M to 0.025 M) of Mg 2+ , which were named as GO HDA-0.25Mg 2+ , GO HDA-0.125Mg 2+ , GO HDA-0.05Mg 2+ , and GO HDA-0.025Mg 2+ , respectively. As shown in Fig. 4a-c, we used GO HDA-Mg 2+ membranes, which have a thickness of about 200 nm, to investigate the permeation with different concentrations of Mg 2+ . As the concentration of Mg 2+ decreased from 0.25 M to 0.025 M, the water uxes gradually reduced from 142.2 to 71.5 L m À2 h À1 bar À1 for methylene blue, from 114.4 to 70.3 L m À2 h À1 bar À1 for pararosaniline, and from 114.2 to 71.6 L m À2 h À1 bar À1 for rhodamine B, respectively, while the rejections remained constant. In the case of the lowest Mg 2+ concentration of 0.025 M, the water uxes were still about seven times that without Mg 2+ . It further indicated that Mg 2+ plays a key role in improving the water ux of cross-linking GO membrane.
In addition, the effect of thickness of membrane on water ux and reject rate were also explored, as shown in Fig. 4d. The thickness of membrane can be controlled by the amount of GO suspension loaded on the substrate. GO HDA-0.25Mg 2+ membranes with four thickness of 100 nm, 200 nm, 300 nm and 400 nm, were prepared by using 1.5 mL, 3 mL, 4.5 mL and 6 mL of mixture GO solution. With increasing of the thickness of the membrane, the water ux decreases dramatically from 251.5 to 90.2 L m À2 h À1 bar À1 while the rejection remained constant, suggesting that the highest water ux could be further improved   16 Hence, the enrichment of Mg 2+ on GO sheets based on the strong cation-p interaction, can promoted reasonable cross-linking and improved the water channels of membrane in terms of atness and low mass transfer resistance. As a result, the GO HDA-Mg 2+ membrane has ultrahigh water ux, while keeping high dyes rejection.

Conclusions
In summary, we successfully enhanced the permeance of the cross-linking GO membrane by using the cation-p interaction between Mg 2+ and aromatic ring structure. The enhancement is positively correlated with the concentration of Mg 2+ . It not only overcomes the swelling of GO membrane and enhances the stability of membrane, but also solves limitation of low water ux. This is attributed to the presence of Mg 2+ , which can prevent excessive chemical cross-linking and improves the water channels of membrane in terms of atness and low mass transfer resistance surface during the cross-linking reaction. Therefore, the GO HDA-0.25Mg 2+ exhibits ultrahigh water ux (143.2 L m À2 h À1 bar À1 ) and high reject rate for dyes in the  ltration experiment. The GO HDA-Mg 2+ membranes also have an outstanding stability in ltration process, which is attributed to the C-N bond formed between GO and HDA. This study suggests that other ions that have strong cation-p interaction, could have a similar effect like Mg 2+ , may opens a new door for the chemical cross-linking mode of GO membrane combined with ions.

Conflicts of interest
There are no conicts to declare.